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The Photodegradation Effect of Metal Oxide-CNT/TiO2 Composites Bull. Korean Chem. Soc. 2011, Vol. 32, No. 3 815

DOI 10.5012/bkcs.2011.32.3.815

The Photodegradation Effect of Organic Dye for Metal Oxide

(Cr2O3, MgO and V2O3) Treated CNT/TiO2 Composites

Ming-Liang Chen, Jang-Soon Bae,† Hee-Seung Yoon,‡ Chang-Sung Lim, and Won-Chun Oh*

Department of Advanced Materials Science & Engineering, Hanseo University, Chungnam 356-706, Korea* E-mail: [email protected]

† Department of Engineering and Chemical Technology, Dankook University, Chungnam 330-714, Korea‡ Department of Chemical Engineering, Chungnam National University, Yuseung, Daejeon 305-764, Korea

Received November 22, 2010, Accepted December 28, 2010

Three kinds of organometallic compounds (chromium acetylacetonate, magnesium acetate and vanadyl

acetylacetonate) were used as transition metal precursor, titanium n-butoxide and multi-walled carbon

nanotube as titanium and carbon precursor to prepare metal oxide-CNT/TiO2 composites. The surface

properties and morphology of metal oxide-CNT/TiO2 composites were by Brauer-Emett-Teller (BET) surface

area measurement, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-raydiffraction (XRD) and energy dispersive X-ray (EDX) analysis. The photocatalytic activity of prepared metal

oxide-CNT/TiO2 composites was determined by the degradation effect of methylene blue in an aqueous

solution under irradiation of visible light.

Key Words : MWCNT, Transition metals, TEM, Photocatalytic activity

Introduction

The degradation of organic pollutants in waste water by

photocatalysis, using the wide optical band gap material

(TiO2), has attracted extensive attentions during recent 20

years.1,2 However, it has been well known that this type of

photo-oxidation has two typical drawbacks: firstly TiO2 is a

high energy band (Eg ≈ 3.2 eV) material that can only be

excited by high energy ultraviolet irradiation with a

wavelength of no longer than 385 nm. This practically rules

out the use of sunlight as an energy source for the photo-

reaction. Secondly, a low rate of electron transfer to oxygen

and a high rate of recombination between excited electron-

hole pairs result in a low quantum yield rate and also a

limited photo-oxidation rate.3 Recently, many studies have

been done to improve photocatalytic properties of TiO2

powders by doping using transition metal elements. The

presence of foreign metal species is generally detrimental forthe degradation of organic species in aqueous systems. Cr

and V ion implanted TiO2 have showed photocatalytic

reactivity higher than TiO2 for the decomposition of NO

under solar beam irradiation.4 Choi et al .5 found that doping

quantum-sized TiO2 with Fe3+, Mo5+, Ru 3+, Os3+, Re5+, V4+

and Rh3+ enhances the photoreactivity both for the oxidation

of CHCl3 and the reduction of CCl4. The photocatalytic

efficiency of TiO2 toward the oxidation of 1,4-dichloro-

benzene is improved by the introduction of WO3 and

MoO36,7 and a beneficial influence of tungsten was found

for the photodegradation of 4-nitrophenol.8,9 Also, in order

to extend the absorption threshold of TiO2 to visible light,the effects of some transition metal ion dopants such as Fe,

V, Mn, Co and Ni have been investigated for the TiO2

system.10

Carbon nanotubes (CNTs) attracted worldwide attention

in the past decade because of their unique structural,

mechanical and electronic conducting properties, corrosion

resistance and stability and promising applications in transi-

stors, field-emission tips, sensors, supercapacitors, catalyst

supports and storage materials for hydrogen.11-13 TiO2/

carbon nanotube (CNT) composites attracted more attention

than others because of the excellent mechanical property,

large surface area, and unique electrical and electronic

properties of CNT.14 According to our previous works,15-17

we prepared the CNT/TiO2 composites by a sol-gel method

and obtained enhanced photocatalytic activity because CNT

could be act as an electron sensitizer and donator to accept

the photo-induced electron (e−) into the conduction band of

TiO2 particles under UV light irradiation.

In this paper, transition metal ion of Cr3+, Mg2+ and V3+

doped CNT/TiO2 composites were synthesized by sol-gel

method. The properties of prepared metal oxide-CNT/TiO2

composites were characterized by BET surface area mea-

surement, scanning electron microscopy (SEM), transmission

electron microscopy (TEM), X-ray diffraction (XRD), and

energy dispersive X-ray (EDX) analysis. Also, the photo-

catalytic properties of metal oxide-CNT/TiO2 composites

were simply checked by decomposing methylene blue (MB)

solution under visible light irradiation. The absorbance of

decomposed MB solution was determined by an UV/VIS

spectrophotometer.

Experimental Procedure

Materials. Titanium n-butoxide (TNB, Ti{OC(CH3)3}4,

99%) as titanium alkoxide precursor to form TiO2 was

purchased from Acros Organics (USA). Multi-walled carbon



816 Bull. Korean Chem. Soc. 2011, Vol. 32, No. 3 Ming-Liang Chen et al.

nanotube (MWCNT, 95.9%) with diameter of ~20 nm and

length of ~5 μm was purchased from Nanokanbon (Korea).

MWCNT was used directly without any purification, but

had to oxidize the surface to obtain more functional groups.

m-chlorperbenzoic acid (MCPBA) was used as strongoxidants to oxidize the MWCNT which obtained from

Acros Organics (USA). Chromium acetylacetonate (Cr(acac)3,

Cr(C5H8O2)3, 97%) and vanadyl acetylacetonate (VO(acac)2,

VO(C5H8O2)2, 98%) with magnesium acetate (Mg(CH3COO)2·4H2O, 99%) were used as transition metal precursors which

purchased from Sigma-Aldrich Chemistry (USA) and Dae-

Jung Chemicals & Metals (Korea), respectively. Benzene

(C6H6, 99.5%), used as a solvent, was purchased from

Samchun Pure Chemical (Korea). Methylene blue (MB,

C16H18 N3S·Cl·3H2O) was used as analytical grade which

purchased from Duksan Pure Chemical Co., Ltd, Korea. The

pure TiO2 and MWCNT/TiO2 photocatalysts were used ascompare materials for photodegradation effect of MB solu-

tion. The pure TiO2, with anatase structure, is obtained from

Duksan Pure Chemical (Korea). The MWCNT/TiO2 photo-

catalyst was prepared in our previous work.15

Preparation of Metal Oxide-CNT/TiO2 Composites. Due

to the MWCNT is very stable, it needs to be treated with

strong acids to introduce active function groups on their

surface. We took 1.0 g MCPBA melted in 60 mL Benzene to

prepare oxidizing agent. And then 0.2 g MWCNT was put

into the oxidizing agent. The mixture was stirred with a

magnet for 6 h at 343 K. Then the MWCNT was dried at

373 K and spared.

Three kinds of organometallic compounds Cr(acac)3,Mg(CH3COO)2 and VO(acac)2 were used to prepare three

kinds of metal oxides. Cr(acac)3 and VO(acac)2 were dissolved

in benzene to prepare 0.01 M Cr(acac)3 and VO(acac)2 solu-

tion. Mg(CH3COO)2 was dissolved in distilled water to

prepare 0.01 M Mg(CH3COO)2 solution. The same amount

of oxidized MWCNT was put into same volume of these

three kinds of solution. And then the solutions were homo-

genized at 343 K for 5 h using a shaking water bath with a

shaking rate of 120 rpm/min. After reaction for 5 h, the

solutions were transformed to the metal oxide-CNT gels,

and these gels were heat treated at 873 K for 1 h with a

heating rate of 279 K/min. Then metal oxide-CNT compositeswere prepared. In a separate preparation, TNB (4 mL) was

dissolved in 46 mL of benzene with constant stirring to form

a TNB-benzene solution. Three kinds of prepared metal

oxide-CNT composites were placed into this solution, respec-

tively. The mixtures were then reacted at 343 K for 5 h using

a shaking water bath at a shaking rate of 120 rpm/min. After

this reaction, the mixtures were treated thermally at 873 Kfor 1 h at a heating rate of 279 K/min. Finally the metal

oxide-CNT/TiO2 composites were obtained. The preparation

condition and code of samples are listed in Table 1. And the

schematics of MWCNT surface oxidation and deposition of

organometallic compound on MWCNT was schematically

illustrated in Figure 1.

Characterization. Synthesized metal oxide-CNT/TiO2

composites were characterized by various techniques. The

BET surface area was measured using a Quantachrome

surface area analyzer (Monosorb, USA). SEM (JSM-5600

JOEL, Japan) and TEM (JEM2000-FX, Japan) were used to

observe the surface state and structure of metal oxide-CNT/ TiO2 composites was carried out. XRD was used for crystal

phase identification and estimation of the anastase-to-rutile

ratio. XRD patterns were obtained at room temperature by

using an X-ray generator (Shimata XD-D1, Japan) using

CuKα radiation. EDX was used to measure the elemental

analysis of metal oxide-CNT/TiO2 composites. The light

absorption spectra of the samples were recorded with an

UV/VIS spectrophotometer (Optizen POP, Mecasys Co.,

Ltd, Korea) in a range of 200-750 nm.

Photocatalytic Activity. The photocatalytic activity of

metal oxide-CNT/TiO2 composites was taken out by de-

composition of MB solution under irradiation of visible

light. In an ordinary photocatalytic test performed at 25 oC,0.05 g photocatalyst was added to 50 mL of 1.0×10−5 mol/L

MB solution and maintained in suspension by magnetic

stirring. After stirring continuously in the dark for 2 h to

ensure establishment of adsorption/desorption equilibrium

Table 1. Nomenclatures of metal oxide-CNT/TiO2

composites

Samples Nomenclatures

MWCNT + 0.01 M Cr(acac)3

solution +

TNB (4 mL)/benzene (46 mL)MCT

MWCNT + 0.01 M Mg(CH3

COO)2

solution +

TNB (4 mL)/benzene (46 mL)MMT

MWCNT + 0.01 M VO(acac)2

solution +

TNB (4 mL)/benzene (46 mL)MVT

Figure 1. Schematics of deposition of organometallic compounds and TiO2

on MWCNT.




of MB, the suspension was irradiated by visible light (8 W,

λ >420 nm, KLD-08L/P/N, Fawoo Technology) and it was

treated as the starting point (t=0) of the reaction, where the

concentration of MB was designated as c0. At specific time

(30 min, 60 min, 90 min and 120 min) intervals a certainvolume of the sample was withdrawn and centrifuged to

remove the catalyst before analysis. The concentration of

MB (c) solution during the photocatalytic degradation reaction

was monitored through measuring the absorbance of the

solution samples with UV/VIS spectrophotometer at λ max

=660 nm.18,19

Results and Discussions

Characterization. Table 2 showed the BET surface area

of pristine MWCNT, MCT, MMT and MVT. The BET

surface area of pristine MWCNT was 299 m

2

/g. Aftertreated by different organometallic compounds and TNB in

benzene solvent, the BET surface area was decreased to

47 m2/g, 39.5 m2/g and 90 m2/g for samples MCT, MMT

and MVT, respectively. It could be considered that the

organometallic compounds and titanium oxide particles

dispersed on the surface of MWCNT, which could clog the

pore of MWCNT, thus decreased the surface area.

The micro-surface structures and morphology of metal

oxide-CNT/TiO2 composites prepared from different organo-

metallic compounds were characterized by SEM and TEM.

Figure 2 showed the SEM images of metal oxide-CNT/TiO2

composites. For sample MCT, the Cr2O3 particles homo-

geneously mixed with MWCNT by TiO2 particles uniformlydistributed on their surface, as showed in Figure 2(a). For

sample MMT, the porous structure could be observed in

Figure 2(b), and the TiO2 agglomerate was coated on the

MgO-CNT composites. For sample MVT, it was difficult to

distinguish the structure of metal, TiO2 and CNT. So we

used TEM to obtain the more detailed observations of

prepared metal oxide-CNT/TiO2 composites.

Figure 3 showed the TEM images of metal oxide-CNT/

TiO2 composites prepared from different organometallic

compounds. For the sample MCT, the Cr2O3 and TiO2

particles were homogenously distributed on the surface of

MWCNT. These structures would be shown the excellent photocatalytic activity. For sample MMT, the TiO2 particles

were distributed on the surface of MWCNT with some

partial agglomerations. For sample MVT, TiO2 particles with

some agglomerates dispersed on the surface of MWCNT

together with V2O3 particles. As we known, a good disper-

sion of small particles could provide more reactive sites for

the reactants than aggregated particles. So it could be

considered that the prepared samples MCT, MMT and MVT

would have good photocatalytic activity for degradation of

MB solution.

The XRD results for the metal oxide-CNT/TiO2 com-

posites prepared from different organometallic compounds

were shown in Fig. 4. Sample MCT showed peaks at 24.5o,

33.6o, 36.2o, 41.4o and 50.5o 2θ due to Cr2O320,21 (JCPDS:

38-1479) and peaks at 25.3o, 37.8o, 48.0o, 53.8o and 54.9° 2θ

due to anatase TiO2 (JCPDS: 21-1272). For sample MMT,

apart from the (111) and (200) diffraction peaks of cubic

MgO (JCPDS: 36-1377) structure from the substrate,22,23 all

recognizable reflection peaks, at 25.3o, 37.8o, 48.0o, 53.8o

and 54.9o, could be well indexed to the anatase TiO2

Table 2. The BET surface area of pristine MWCNT, MCT, MMTand MVT

Samples SB E T

(m2 /g)

Pristine MWCNT 299

MCT 47

MMT 39.5

MVT 90

Figure 2. SEM images of metal oxide-CNT/TiO2

compositesprepared from different organometallic compounds; MCT (a),MMT (b) and MVT (c).




structure. In sample MVT, the peaks at 2θ=36.2o, 41.2o due

to V2O324,25 (JCPDS: 34-0187) and peak at 2θ = 27.4o due to

rutile structure of TiO2 (JCPDS: 21-1276) and peaks at 2θ =

48.0o

and 54.9o

due to anatase structure of TiO2. It could beindicated that after heat treatment at 873 K for 1 h, the

organometallic compound precursors Cr(acac)3, Mg(CH3COO)2,

VO(acac)2 and TNB have been changed to Cr2O3, MgO,

V2O3 and TiO2. The intensity of TiO2 was decreased by an

order of MMT, MCT and MVT, indicated the TiO2 content

in composites was also decreased by an order of MMT,

MCT and MVT. It could be also observed that no reflection

peaks from impurities existed in XRD patterns for all of

samples, indicating the high purity of the products. On the

other hand, the characteristic peaks of MWCNT could

hardly be identified from the XRD patterns of all samples. It

was thought that the absence of MWCNT aggregated pores

was supported by the disappearance of CNTs characteristic

peaks in XRD patterns.

EDX was conducted on several zones of metal oxide-

CNT/TiO2 composites prepared from different organometallic

compounds. The main elements found in a representative

analysis were shown in Figure 5. Three kinds of main

elements C, O and Ti were existed in all of samples and

metal element Cr, Mg and V were existed in samples MCT,

MMT and MVT, respectively, and without any other impure

elements. However, the Ti content in sample MMT was

much more than that in samples MCT and MVT. This result

was agreed with the results of XRD which the intensity of

TiO2 was strongest in sample MMT among these three kinds

of samples.

Figure 3. TEM images of metal oxide-CNT/TiO2

compositesprepared from different organometallic compounds; MCT (a),MMT (b) and MVT (c).

Figure 4. The XRD patterns of samples MCT, MMT and MVT.

Figure 5. EDX elemental microanalysis of metal oxide-CNT/TiO2

composites prepared from different organometallic compounds;MCT (a), MMT (b) and MVT (c).




Photocatalytic Activity. Figure 6 showed MB removal

by photocatalytic degradation for pure TiO2, CNT/TiO2

composites, MCT, MMT and MVT under irradiation of

visible light for 120 min. As mentioned above, the TiO2

could only show photocatalytic activity under UV light, due

to its wide band gap (3.2 eV for anatase), and did not act

with the solar light effectively. So in the present study, pure

TiO2 shows a little of photocatalytic activity only decreased

3.4% of MB solution under visible light after 120 min. For

CNT/TiO2 composite, after irradiation for 120 min under

visible light, the concentration of MB solution was decreas-

ed 15%, more than pristine TiO2. For metal oxide-CNT/TiO2

composites, they showed much more photocatalytic activity

than CNT/TiO2 composite. And the concentration of MB

solution was decreased 48%, 32% and 46% for samples

MCT, MMT and MVT, respectively. In addition, the kinetic

plots were shown by apparent first-order linear transform

−ln(c/c0) against time function f (t ) in Figure 7. Table 3

showed the apparent kinetic constant (k app) of pure TiO2,

CNT/TiO2 composites MCT, MMT and MVT. The k appof the pure TiO2 and CNT/TiO2 composites was 4.38×10−4

min−1 and 9.94×10-4 min−1. However, the k app of metal

oxide-CNT/TiO2 composites was much higher than that of

pure pure TiO2 and CNT/TiO2 composites, which were3.92×10−3 min−1, 3.22×10−3 min−1 and 3.73×10−3 min−1 for

samples MCT, MMT and MVT, respectively. The introduc-

tion of MWCNT and metal oxide into matrix obviously

created kinetic combination effect in MB degradation with

an increase in the rate constant by the combination factor of

8.9, 7.35 and 8.5 for samples MCT, MMT and MVT,

respectively. It could be indicated that the metal oxide-CNT/

TiO2 composites had more photocatalytic activities under

irradiation of visible light region.

The photocatalytic activity of TiO2 could be controlled by

the following factors: (i) light absorption wavelength; (ii)

rate of the electron or hole induced redox reaction; and (iii)

recombination of the electron-hole. The mechanism of

photodegradation of dye solution for metal oxide-CNT/TiO2

composites was shown in Figure 8. When a transition metal

ion (Cr3+, Mg2+ or V3+) was incorporated into the TiO2

lattice, the dopant level appears between the valence band

and conduction band of TiO2,21,26,27 thereby altering the

band-gap energy and shifting the absorbance edge to the

visible light region. According to previous studies, MWCNT

could act as an electron sensitizer and donor in the

composite photocatalyst to accept a photo-induced electron

(e−) into the conduction band of TiO2 particles under light

irradiation, thereby increased the number of electrons as

well as the rate of electron-induced redox reactions. The

Figure 6. MB removal by photocatalytic degradation for pureTiO

2

, CNT/TiO2

composites, MCT, MMT and MVT underirradiation of visible light for 120 min. Figure 7. Apparent first-order linear transform −ln(c/c

0

) against

(t ) of MB degradation kinetic plots for pure TiO2

, CNT/TiO2

composites, MCT, MMT and MVT.

Table 3. The apparent kinetic constant (k a p p

) and combinationfactor (R) of pure TiO

2

, CNT/TiO2

composites, MCT, MMT andMVT

Samples k a p p

(min− 1 ) R

Pure TiO2

4.38×10− 4 1

CNT/TiO2

9.94×10− 4 2.26

MCT 3.92×10− 3 8.9

MMT 3.22×10− 3 7.35

MVT 3.73×10− 3 8.5

Figure 8. A prevenient mechanism for the metal oxide-CNT/TiO2

composites.




addition of a transition metal also had a charge trapping

effect. Charge trapping can be demonstrated by the follow-

ing equations:5

TiO2 + hv →

ecb−

+ hvb

+

(1)

Mn+ + ecb− →M(n−1)+ (2)

Mn+ + hvb+ →M(n+1)+ (3)

The holes could transfer to the TiO2 surface and react with

OH− to produce active OH•. When a transition metal ion

replaced Ti ions in the TiO2 lattice, most of the dopant levels

appeared between the valence band and conduction band of

TiO2. This could increase the surface trapping rate of the

carrier and retard the electron-hole recombination23,28 as

well as enhance the photocatalytic activity of TiO2. Finally,

the MB solution is decomposed to CO2, H2O, NO3, NH4+

and SO42−.

Conclusions

We introduced transition metals into CNT/TiO2 com-

posites to prepare metal oxide-CNT/TiO2 composites by

using three kinds of organometallic compounds (Cr(acac)3,

Mg(CH3COO)2 and VO(acac)2). The BET surface area was

decreased a lot after treatment by organometallic compounds

and titanium for all of metal oxide-CNT/TiO2 composites.

For the sample MCT, the Cr2O3 and TiO2 particles were

homogenously distributed on the surface of MWCNT. For

sample MMT, the TiO2 particles were distributed on thesurface of MWCNT with some partial agglomerations. For

sample MVT, TiO2 particles with some agglomerates dis-

persed on the surface of MWCNT together with V2O3

particles. From the XRD results, Cr2O3, MgO and V2O3

structures were exited in samples MCT, MMT and MVT,

respectively. The anatase type TiO2 structures were also exit-

ed in samples MCT and MMT, and a mixture strcutures of

anatase and rutile type TiO2 were exited in sample MVT.

Three kinds of main elements (C, O and Ti) were exited in

all of metal oxide-CNT/TiO2 composites, and element Cr,

Mg and V was exited in samples MCT, MMT and MVT,

respectively. Comparison with pure TiO2 and CNT/TiO2composites, the prepared metal oxide-CNT/TiO2 composites

showed very high photocatalytic degradation efficiency for

MB solution under visible light irradiation. Because the

transition metal ions could incorporate into the latice of

TiO2, alter the band-gap energy and shift the absorbance

edge of TiO2 to the visible light region.

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the photodegradation effect of organic dye for cr2o3 treated cnt-tio2

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